In many applications, semiconductor devices are packaged within a molded body for protection against the corrosive elements of the environment. A plurality of lead conductors or leads project from the sides of the molded body for connecting the semiconductor device to an associated circuit such as a circuit board. The parallel leads project from the molded body in a predetermined configuration.
The lead conductors are constructed from a pliable metal which can be easily formed into a desired configuration without damaging either the semiconductor chip or the molded body. However, the application of even a small amount of pressure easily bends the leads away from the desired configuration. Considerable care must be taken when handling the packaged semiconductor device to protect the lead conductors from deformation. Even then, the leads of a large number of semiconductor devices are bent away from the predetermined configuration.
The deformed leads of a semiconductor device may have been pressed close together, removing the preformed parallelism, or bent vertically relative to the molded body so that they do not lie on a common plane. The tips of the bent leads will not meet the correct points on the circuit board when the semiconductor device is mounted in place. The lead conductors on opposing ends of the molded body may be bent, distorting the tweeze or tip-to-tip distance of the device. Because of this distortion, the semiconductor device will not fit within the designated location on the circuit board.
One method of mounting a semiconductor device to a circuit board includes positioning the device by hand and then soldering it in place. More commonly, the semiconductor device is set on the circuit board by an automatic placement machine, with the tips of the leads positioned on a spot of solder paste. The board is then heated, melting the solder paste to secure the leads in place and electrically connect the semiconductor device to other components. If the tips of pressed-together leads are not equally spaced about the perimeter of the molded body, some of the leads will rest directly on the circuit board. Leads bent vertically relative to the molded body will not all contact the surface of the circuit board. Lead tips which are suspended above the board by more than six mils will not touch the solder paste.
Lead conductors which do not contact a spot of solder paste will not be secured in place when the board is tested. The loose tips do not electrically connect the semiconductor device to the other components, resulting in a defective circuit board. The spots of solder paste not touching a lead tip tend to bridge the pads on the board and short the electronic circuit. Thus, a semiconductor device with bent leads is defective and may not be used. Since substantial expense is involved in the manufacture of semiconductor devices, a system which would reform the bent leads to the desired configuration is desirable.
Methods exist in the art for salvaging some of the defective semiconductor devices by straightening the bent leads. The bent lead conductors can be reformed by manually bending each lead back to the appropriate configuration. Using this method does restore many of the deformed devices to an acceptable condition, however the process is extremely time consuming and subject to human error. Additionally, the reformed leads retain an elastic memory of the deformation and tend to partially spring back to the bent condition.
If not severely deformed, the bent lead conductors may be reformed by spanking the semiconductor device or compressing the projecting leads between opposing dies. Compressing the lead conductors will not reintroduce parallelism between the leads. In a slight modification, one of the opposing dies includes teeth to separate the lead conductors. An example of a device implementing this method is disclosed in U.S. Pat. No. 4,727,912, issued to Gonzalez. In Gonzalez, teeth mounted to an upper die are inserted in between the leads of a semiconductor device positioned on a lower die as the upper die is lowered. The upper and lower dies are pressed together, compressing and indenting the leads to remove the elastic memory of the deformation.
Spanking the semiconductor device will reform the lead conductors which have been bent vertically relative to the molded body. However, compressing the leads will only partially remove the elastic memory of the deformation. Once removed from the spanking device, the leads will partially return to the bent condition. Mounting teeth to the upper die as in Gonzalez will reintroduce parallelism between the lead conductors, but severely deformed leads may be damaged by the teeth when the dies are pressed together. Moreover, the leads will spring away from the reintroduced parallelism toward the bent position. A system for reconditioning bent leads which is capable of reintroducing parallelism and the predetermined configuration while substantially erasing lead memory of prior bending is desirable.
An alternative method for conditioning the deformed leads uses opposing blades to straighten and massage the leads as the semiconductor device passes along a track, as is disclosed in U.S. Pat. No. 4,481,984, issued to Linker. In Linker, two wiper blades and an opposing separator blade having a number of grooves are used to separate the leads depending below a semiconductor device supported on a track. Ridges separating the grooves on the separator blade partially engage the less damaged leads and force any crossed leads towards the wiper blades. The first wiper blade directs the crossed leads from the ridges to the grooves, after which the second wiper blade pushes the leads into the grooves and holds them in place. The separator blade and the second wiper blade are oscillated to massage the compressed leads.
Reforming the leads by using opposing blades as in Linker will reintroduce parallelism to the leads while partially removing elastic memory of the deformation. Compressing the lead conductors between the blades reforms the leads which have been bent vertically relative to the molded body of the semiconductor device. However, as in Gonzalez, elastic memory of the deformation is not completely removed by compressing the leads. After the conditioning, the lead conductors will tend to return to the bent condition. Additionally, severely bent leads may be damaged as the blades are pressed together. A system for reconditioning bent leads which reintroduces parallelism and a predetermined configuration while completely erasing elastic memory of the prior deformation is highly desirable.
The device disclosed in Gonzalez will only reform leads projecting from one side of a semiconductor device. Similarly, a device using opposing blades as is disclosed in Linker reforms lead conductors projecting from only two sides of a semiconductor device at the same time. A system which offers the efficiency of simultaneously reconditioning the leads projecting from all four sides of a semiconductor device would be particularly valuable.